Get all Fridays in a date Range in Java

Question

I recently came across a task where i have to get all Fridays in a date range. I wrote a small piece of code and was surprised see some strange behaviour.

Below is m

Answer 1

Here a solution based on new stream-features of Java-8 and using my library Time4J (v4.18 or later):

String start = "01/01/2009";
String end = "12/09/2013";

ChronoFormatter f =
    ChronoFormatter.ofDatePattern("dd/MM/yyyy", PatternType.CLDR, Locale.ROOT);

PlainDate startDate =
    f.parse(start).with(PlainDate.DAY_OF_WEEK.setToNextOrSame(Weekday.FRIDAY));
PlainDate endDate = f.parse(end);

Stream fridays =
    DateInterval.stream(Duration.of(1, CalendarUnit.WEEKS), startDate, endDate);

fridays.forEachOrdered(System.out::println);

// output
2009-01-02 
2009-01-09 
... 
2013-08-30 
2013-09-06

// other example: list of fridays in ISO-8601-format
List result =
  DateInterval.between(startDate, endDate)
    .stream(Duration.of(1, CalendarUnit.WEEKS))
    .map((date) -> date.toString()) // or maybe use dd/MM/yyyy => f.format(date)
    .collect(Collectors.toList());

By the way, Java-9 will offer a similar solution (but with exclusive end date boundary), see also this enhancement-issue.